EXCLUSIVE

Faye Resnick -- Kyle Richard's best friend and who regularly appears on "Real Housewives of Beverly Hills," claims a strange person has been following her and even left her a threatening letter.

Law enforcement tells TMZ, Resnick -- who also wrote a book accusing OJ Simpson of murdering her then-best friend Nicole Brown SImpson -- called police to report someone was tailing her.

Resnick -- who has also modeled for Playboy -- told cops someone even snuck on to her property and plastered a threatening note on her car.

We're told police are on the hunt but don't have a suspect.

In the meantime, cops have been doing extra patrols on Faye's block.